Lanthanide-doped layered double hydroxides and method of producing same

ABSTRACT

The present disclosure relates to a method for producing lanthanide doped layered double hydroxides (Ln-doped LDHs). The method includes the steps of preparing a carbonate free alkaline solution; preparing a solution of metal salts comprising a salt of a lanthanide; co-precipitating the alkaline solution and the solution of metal salts to form a mixture and Ln-doped LDH precipitate wherein the pH of the mixture is maintained at a constant value; aging the precipitate; and separating the precipitate from the solution. The alkaline solution is an aqueous ammonia solution. The present disclosure is also related to lanthanide-doped layered double hydroxides (La-doped LDHs) obtainable by such a method, as well as to the use of the lanthanide-doped layered double hydroxides obtainable by such a method.

TECHNICAL FIELD

The present disclosure is related to a method for producinglanthanide-doped layered double hydroxides (Ln-doped LDHs). The presentdisclosure is also related to lanthanide-doped layered double hydroxides(La-doped LDHs) obtainable by such a method, as well as to the use ofthe lanthanide-doped layered double hydroxides obtainable by such amethod.

INTRODUCTION

Heavy metals such as Cr, Mo, W, Mn, V, Nb, Sb, etc. are found inlow-grade industrial waste streams such as hydrometallurgical slags.These industrial waste streams are complex mixtures in which theby-product metals are usually present in low to very low amountscompared to the associated major metals. In the art, metal liberation byalkaline leaching is typically performed on waste streams, generatingcomplex leachate mixtures with high pH (e.g. pH from 12 to 14)containing the valuable metals. Further treatment of these leachatemixtures is then necessary in order to separate, recover or remove themetals therefrom. However, as the pH of the leachate mixtures is high,any conventional recovery or removal treatment first requires thereduction of the pH of these mixtures to the acidic range (e.g. to a pHbelow 5, typically around 2 to 3) before actually proceeding with thefurther separation, recovery or removal of the metals. This pH reductionis often a challenging and costly operation.

The state-of-the-art regarding oxyanion uptake include sorbents ofdifferent nature or provenience, such as activated carbons, anionicexchange resins, bio-materials, waste products, or minerals, themajority thereof having low stability and/or low performance (i.e.limited sorption capacity) in alkaline environment (i.e. at pH higherthan 7). For example, anionic exchange resins (such as Purolite® A830,Lewatit® MP 62, Lewatit® M 610, Lewatit® MP 64), only efficiently workat a pH lower than 7 (typically in a pH range of 1 to 4). Moreover, thesorption equilibrium for such resins is only reached after about 2hours, the uptake of heavy metal oxyanions hence being very slow. Andeven in the acidic conditions (pH lower than 7), the maximum sorptioncapacity of many of the anionic exchange resins known in the art isabout 40 mg of heavy metal oxyanions per gram of sorbent, which isrelatively low.

Recently, there is an increasing interest in using a class of anionicclays known as layered double hydroxides (LDHs), or hydrotalcite-likecompounds (HTlc), to remove inorganic contaminants such as oxyanions andmonoatomic anions (e.g. fluoride, chloride, bromide, and iodide) fromaqueous solutions by a process of adsorption and/or ion exchange. Goh etal. in Water Res. 2008, 42(6-7), 1343-68, for example, provides anoverview of LDH synthesis methods, LDH characterization techniques, andapplication of LDHs for removal of oxyanions, as generally known in theart up to now.

LDHs are mineral materials having a layered clay-type structure. Thestructure of LDH materials arises from the structure of brucite, themineral form of magnesium hydroxide with chemical formula Mg(OH)2, inwhich divalent metal cations M2+ (said metal cations being octahedrallycoordinated by hydroxyl groups) are isomorphously substituted by highervalence cations M3+ (such as Al3+). This substitution is generatingpositively charged brucite-type layers which are electro-neutralized byanions which, together with crystallization water molecules, are locatedin the interlayer region.

The structure of a typical octahedral unit and of a LDH are shown inFIG. 1 and FIG. 2, respectively. The basal spacing (c′) depicted byreference sign (1) in FIG. 2 is the total thickness of the brucite-likesheet (3) and the interlayer region (2) of the LDH structure. Theoctahedral units of M2+ or M3+ (being sixfold coordinated to OH—) shareedges to form infinite sheets. These sheets are stacked on top of eachother and are held together by hydrogen bonding.

LDHs are represented by the general formula As generally known, LDHmaterials are endowed with excellent anion exchange abilities for a highnumber of inorganic anions (e.g. nitrate, sulphate, chromate, etc.) andorganic anions (e.g. acetate, tartrate, carboxylate, etc.). Furthermore,LDHs are known in the art to possess thermal stability, having arelative high surface area, being easy and economically viable toproduce, and having low toxicity. This versatility of the LDH-typematerials has already led to their application in many fields, forexample for use as sorbents for (oxy)anions, as catalysts, catalystsprecursors or catalytic supports, as nano-fillers for polymericnanocomposites, as sensors, or in medicine as drug-carriers. However, amain concern in the application of LDHs as sorbent for (oxy)anions inthe art is still related to their low stability in alkaline environment.

Accordingly, there is a desire to provide LDHs being stable at high pH(e.g. ranging from 10 to 13) while retaining maximal sorption capacity.

Poernomo Gunawan et al., “Lanthanide-Doped Layered Double HydroxidesIntercalated with Sensitizing Anions: Efficient Energy Transfer betweenHost and Guest Layers”, Journal of Physical Chemistry C, vol. 113, No.39, pp. 17206-17214 describes a process for the preparation of Terbium(Tb)-doped layered double hydroxides comprising the steps of adding amixed solution of salts of Mg, Al and Tb drop-wise to an aqueousNaOH-solution, aging and separating the precipitate.

It has been observed by the present inventors that Ln-doped LDH obtainedby the above route suffer from decrease of crystallinity with increaseddegree of Ln-doping.

SUMMARY

An objective of aspects of the present disclosure is to provide at leastan alternative route for the preparation of lanthanide-doped layereddouble hydroxides (LDHs), in particular to provide a preparation routethat does not have the above mentioned drawbacks and/or which is morerobust and/or which leads to improved lanthanide-doped LDHs. It is anobjective of aspects of the present disclosure to produce layered doublehydroxide-type anionic clays and to use the produced materials assorbent.

According to first aspects of the disclosure, there is thereforeprovided a method for producing lanthanide-doped layered doublehydroxides (LDHs) as set out in the appended claims. The methodcomprises the steps of preparing an alkaline solution, wherein thealkaline solution does not comprise carbonates; preparing a solution ofmetal salts comprising a salt of a lanthanide; combining the alkalinesolution and the solution of metal salts so as to form a mixture andLn-doped LDH precipitate wherein the pH of the mixture is maintained ata constant value; aging the precipitate; and separating the precipitatefrom the solution. Performing the method is free from carbonates.According to the present disclosure, the alkaline solution is an aqueousammonia solution.

By performing the method of the present disclosure, the doping of theLDH material with lanthanides is performed within the brucite-likesheets (and not by incorporating the lanthanides in the interlayerregion between two brucite-like sheets, as it is the case using methodsknown in the art). Furthermore, the preparation route according to thepresent disclosure allows for reducing or even completely preventinglanthanide segregation. Alkaline stability is thereby provided to theLn-doped LDHs obtainable by methods of the present disclosure, allowingthe doped LDHs to be directly applied as sorbent for recovering(oxy)anions in high alkaline environment (i.e. having a pH of 10 up toaround 13.5).

Further advantageous aspects of the present disclosure are set out inthe dependent claims.

According to second aspects of the present disclosure, there is providedlanthanide-doped layered double hydroxides, such as obtainable orobtained by methods according to the present disclosure, as set out inthe appended claims. In the lanthanide-doped LDHs, the lattice parametera₁₁₀ of the unit cell of the crystal structure of the LDH material isadvantageously increased with at least 1.6% compared to the latticeparameter a₁₁₀ of the unit cell of the crystal structure of thenon-doped LDH material. The increase in a₁₁₀ confirms that thelanthanides are incorporated directly in the lattice layers of the LDHmaterial (and not in the interlayer region between two brucite-likesheets, as it is the case for doped LDHs produced using methods known inthe art).

According to yet other aspects of the present disclosure, there isprovided the use of lanthanide-doped layered double hydroxidesobtainable by a method according to the first aspects, or oflanthanide-doped LDH according to the second aspect, as set out in theappended claims. The use as sorbent, as catalyst, or as fluorescentmaterial is described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure will now be described in more detailwith reference to the appended drawings, wherein same reference numeralsillustrate same features and wherein:

FIG. 1 schematically represents an octahedral unit with M²⁺ or M³⁺ metalcations being octahedrally coordinated by OH⁻ anions;

FIG. 2 schematically represents a layered double hydroxide (LDH)structure with (1) the basal spacing (c′), (2) the interlayer region,and (3) the brucite-like sheet;

FIG. 3 depicts a XRD pattern (left) and SEM image (right) of Ca2Alhydrocalumite-type structure obtained in Example 1.A (non-La dopedHC-type material);

FIG. 4 depicts XRD patterns of (a) Ca2Al, (b) Ca3Al and (c) Ca4Alhydrocalumite-type materials obtained in Example 1.A (non-La dopedHC-type material);

FIG. 5 depicts XRD patterns of (a) Ca2Al; (b) Ca2Al0.9La0.1 (3.3 m %La), (c) Ca2Al0.8La0.2 (6.6 m % La), (d) Ca2Al0.5La0.5 (16.5 m % La) and(e) La(OH)₃ reference material (Example 1.B, La-doped HC-type material);

FIG. 6 depicts XRD patterns of Ca2Al0.9La0.1 (3.3 m % La) aged (a) atRT, (b) at 65° C., (c) at 110° C. and (d) La(OH)₃ reference material(Example 1.B, La-doped HC-type material);

FIG. 7 depicts XRD patterns of (a) Mg3Al (Example 1.C, non-La dopedHT-type material); (b) Mg3Al0.95La0.05 (1.65 m % La) and (c)Mg3Al0.9La0.1 (3.3 m % La) (Example 1.D, La-doped HT-type material);

FIG. 8 shows the adsorbed chromate amounts (q_(e), mgCr⁶⁺/g) at pH 8 asa function of time for conventional non-La doped hydrotalcite-type (LDH)materials having different intercalated anions: ZnAl-type (left) andMgAl-type (right), compared with PURAL® MG 63 HT;

FIG. 9 shows the sorption efficiency of chromate and material stabilityduring sorption tests at different pH for ZnAl-type (left) and MgAl-type(right) conventional hydrotalcites;

FIG. 10 depicts cation speciation as a function of pH in Pourbaixdiagrams (E_(h)-pH diagrams);

FIG. 11 illustrates the sorption efficiency during sorption tests atdifferent pH (left) and measurements of metal evolution during sorptiontests as a function of the pH (right);

FIG. 12 depicts XRD patterns of La-doped MgAl HT-type material beforeand after chromate sorption in different pH media: (a) Mg3Al0.9La0.1(3.3 m % La) as synthesised (cf. Example 1.D, La-doped HT-typematerial), (b) Mg3Al0.9La0.1 (3.3 m % La) after sorption at pH 8 and (c)Mg3Al0.9La0.1 (3.3 m % La) after sorption at pH 13;

FIG. 13 represents the volume of ammonia solution required for achievingdifferent pH of co-precipitation step during synthesis at differenttemperatures;

FIG. 14 represents XRD patterns of (a) Mg3Al0.95La0.05 (1.65 m % La),(b) Mg3Al0.95Ce0.05 (1.65 m % Ce), (c) Mg3Al0.95Eu0.05 (1.65 m % Eu),(d) Mg3Al0.95Tb0.05 (1.65 m % Tb), (e) Mg3Al0.95Gd0.05 (1.65 m % Gd) and(f) Mg3Al0.95Yb0.05 (1.65 m % Yb);

FIG. 15 represents Raman spectra of the Mg3Al0.95Eu0.05 (1.65 m % Eu)and the inset is an enlarged view in the spectral range below 1200 cm⁻¹.

DETAILED DESCRIPTION

Methods for producing lanthanide doped layered double hydroxides(Ln-doped LDHs) according to aspects of the present disclosure includethe steps of:

(a) preparing an alkaline solution (as co-precipitation agent), whereinthe alkaline solution does not comprise carbonates;

(b) preparing a (aqueous) solution of metal salts (as precipitationsolution) comprising a salt of a lanthanide;

(c) combining the alkaline solution and the solution of metal salts soas to form (by co-precipitation) a mixture of Ln-doped LDH precipitateand solution thereby maintaining the pH of the formed solution at aconstant value;

(d) aging the precipitate; and

(e) separating the precipitate from the solution.

The synthesis according to the method of the present disclosure is freefrom carbonates. More particularly, the co-precipitation in aspects ofthe present disclosure is performed in alkaline environment in theabsence of carbonates (e.g. Na₂CO₃) as co-precipitating agent (i.e. thealkaline solution is free from carbonates, or, in other words, thealkaline solution does not comprise carbonates). Moreover, methods inthe art for producing Ln-doped LDHs fail to incorporate the Ln³⁺ cationswithin the brucite-like sheets, a requisite to retain the ion-exchangecharacteristic and a key in providing the alkaline stability of thedoped structure (cf. further below).

Indeed, for example Wang et al. in Chemical Engineering Journal, 2017,309, 445-453 report the synthesis of MgAl- and CaAl-containingstructures comprising La³⁺ metal as dopant, said structures to be usedas a coagulation agent for graphene oxide. The synthesis of thestructures reported by Wang et al. is performed via co-precipitation ofmetal nitrate salts in the presence of NaOH and Na₂CO₃ mixture whichfavours the formation of carbonate species in the very early stage ofco-precipitation, thus impeding the isomorphous substitution into thelattice. In fact, the reported structure for the MgAlLa material by Wanget al. reveals a material composed of LDH phase combined with asegregated La₂O₃ phase (the latter phase being a major phase due to thehigh La content with respect to Al), thus a composite LDH/La₂O₃ ratherthan a La-doped MgAl structure. Moreover, the reported structure by Wangcontains carbonate anions in the interlayer. As will be furtherdemonstrated in the Examples below, the Ln-doped LDH material obtainableby a method of the present disclosure is composed of only a LDHstructure in which the Ln³⁺ cations (advantageously the La³⁺ cations)are incorporated or doped (by isomorphous substitution) in the latticelayers (as can be observed from the XRD patterns of FIG. 7 and FIG. 12,and the data in Tables 1A to 1D, cf. examples below). In methodsdescribed in the art, on the contrary, La-doped LDHs are only obtainedby intercalation of lanthanide anionic complexes in the interlayerregion of the LDHs.

According to aspects of the present disclosure, the alkaline solution isan ammonia solution. Advantageously, the concentration of the ammoniasolution is comprised between 20 and 30% (w/w) NH₃, advantageously theconcentration of the solution is 25% (w/w) NH₃.

In the context of the present disclosure, an ammonia solution refers toa solution of ammonia in water, denoted by NH₃(aq).

Advantageously, the (aqueous) solution of metal salts comprises (orconsists of) a salt of a lanthanide, (a salt of) aluminium and (a saltof) one or more of calcium, magnesium, and zinc.

In the context of the present disclosure, a salt of a lanthanide refersto a salt comprising as cation one of the elements of the lanthanides.

Advantageously, the lanthanide in the solution of metal salts islanthanum (La) (or, in other words, the solution of metal saltscomprises a salt of lanthanum), Europium (Eu), or Terbium (Tb).

Advantageously, the solution of metal salts is free from carbonates, or,in other words, the solution of metal salts does not comprisecarbonates.

More advantageously, the solution of metal salts comprises (or consistsof) a salt of lanthanum, (a salt of) aluminium and (a salt of) one ormore of calcium, magnesium, and zinc.

More advantageously, the solution of metal salts comprises (or consistsof) a salt of a lanthanide, (a salt of) aluminium and (a salt of)calcium, magnesium, or zinc. Even more advantageously, the solution ofmetal salts comprises (or consists of) a salt of lanthanum, (a salt of)aluminium and (a salt of) calcium, magnesium, or zinc.

More advantageously, in the solution of metal salts the molar ratio ofCa/Al/Ln (advantageously Ca/Al/La) is 2 to 4/0.5 to 0.95/0.05 to 0.5, orthe molar ratio of Mg/Al/Ln (advantageously Mg/Al/La) is 2 to 4/0.5 to0.95/0.05 to 0.5, or the molar ratio of Zn/Al/Ln is 2 to 4/0.5 to0.95/0.05 to 0.5. Advantageously, the molar ratio of Mg/Al/Ln(advantageously Mg/Al/La) is 2 to 4/0.9 to 0.95/0.05 to 0.1. Even moreadvantageously, in the solution of metal salts the molar ratio ofCa/Al/Ln (advantageously Ca/Al/La) is 2/0.5 to 0.9/0.1 to 0.5, or themolar ratio of Mg/Al/Ln (advantageously Mg/Al/La) is 3/0.9 to 0.95/0.05to 0.1, or the molar ratio of Zn/Al/Ln (advantageously Zn/Al/La) is2/0.5 to 0.95/0.05 to 0.5.

Advantageously, the anions present in the (aqueous) solution of metalsalts are hydroxide (OH⁻), and nitrate (NO³⁻), bromide (Br), chloride(Cl⁻), or fluoride (F⁻); advantageously the anions present in thesolution of metal salts are hydroxide, and nitrate or chloride. In amethod of the present disclosure, the anions present in the solution ofmetal salts form the interlayer anions in the interlayer region betweenthe brucite-like sheets of the Ln-doped LDH material. Advantageously atleast 70% of the anions are nitrate, advantageously at least 80%,advantageously at least 90%.

More advantageously, the solution of metal salts comprises (or consistsof) CaCl₂.2H₂O (calcium chloride dihydrate), AlCl₃.6H₂O (aluminiumchloride hexahydrate) and LaCl₃.7H₂O (lanthanum(III) chlorideheptahydrate). Alternatively, the solution of metal salts comprises (orconsists of) Mg(NO₃)₂.6H₂O (magnesium nitrate hexahydrate),Al(NO₃)₃.9H₂O (aluminium nitrate nonahydrate) and La(NO₃)₃.6H₂O(lanthanum(III) nitrate hexahydrate). In yet another alternative, thesolution of metal salts comprises (or consists of) Zn(NO₃)₂.6H₂O (zincnitrate hexahydrate), Al(NO₃)₃.9H₂O (aluminium nitrate nonahydrate) andLa(NO₃)₃.6H₂O (lanthanum(III) nitrate hexahydrate).

More advantageously, Ln-doped Ca²⁺ containing LDH material (i.e.Ln-doped hydrocalumites) obtainable by a method of the presentdisclosure comprise chlorides (i.e. the chlorides initially beingpresent in the solution of metal salts) as interlayer anions in theinterlayer region between the brucite-like sheets. Alternatively,Ln-doped Mg²⁺ containing LDH material (i.e. Ln-doped hydrotalcites)obtainable by a method of the present disclosure comprise nitrates (i.e.the nitrates initially being present in the solution of metal salts) asinterlayer anions in the interlayer region between the brucite-likesheets. The nitrates advantageously form at least 90% of the anions inthe interlayer region. In yet another alternative, Ln-doped Zn²⁺containing LDH material (i.e. Ln-doped zaccagnaite) obtainable by amethod of the present disclosure comprise nitrates (i.e. the nitratesinitially being present in the solution of metal salts) as interlayeranions in the interlayer region between the brucite-like sheets.

Even more advantageously, the concentration of the solution of metalsalts is comprised between 0.5M and 2M, advantageously being 1M.

Even more advantageously, in the solution of metal salts comprisingCaCl₂.2H₂O, AlCl₃.6H₂O and LaCl₃.7H₂O, the mole percent (or molarpercentage or molar proportion, mol %) of La is comprised between 1 to17.5, advantageously between 3.3 to 16.5. Alternatively, in the solutionof metal salts comprising Mg(NO₃)₂.6H₂O, Al(NO₃)₃.9H₂O andLa(NO₃)₃.6H₂O, the mole percent of La is comprised between 1 to 17.5,advantageously between 1.65 to 17.5, advantageously between 1.65 to 3.3.In yet another alternative, in the solution of metal salts comprisingZn(NO₃)₂.6H₂O, Al(NO₃)₃.9H₂O and La(NO₃)₃.6H₂O, the mole percent of Lais comprised between 1 to 17.5, advantageously between 1.65 to 17.5.

The metal salts in the precipitation solution (i.e. in the solution ofmetal salts) are the precursors for forming the precipitate byco-precipitation with the alkaline solution in step (c). Moreparticularly, by combining, in step (c), the alkaline solution and thesolution of metal salts, a mixture of Ln-doped LDH precipitate andsolution is formed by co-precipitation (more particularly, Ln-doped LDHprecipitate (solid phase) in solution is formed). The alkaline solutionand the solution of metal salts are combined in such a way that the pHof the solution formed in said step is maintained at a constant value.In this way, the doping of the LDH material with lanthanides, moreparticularly with Ln³⁺ cations (advantageously with lanthanum, moreparticularly with La³⁺ cations) is performed by isomorphous substitutionof the trivalent metal cations M³⁺, initially being present in theoctahedral units in the brucite-like sheets of the LDH material, with(or by) the Ln³⁺ cations (advantageously the La³⁺ cations). In aspectsof the present disclosure, the chemical composition of the LDH materialis thus tuned by doping with Ln³⁺ cations (advantageously La³⁺ cations)within the brucite-like sheets (and not by incorporating the cations inthe interlayer region between two such brucite-like sheets). The dopingcan be performed in various cationic ratios. This way of doping providesalkaline stability to the Ln-doped LDHs (advantageously La-doped LDHs)produced by methods of the present disclosure, allowing the Ln-dopedLDHs to be directly applied as sorbent in harsh conditions, e.g. for thedirect use for sorption of valuable metals from industrial complex highalkaline leachate mixtures having a pH of 10 up to around 13.5.

Combining the alkaline solution and the solution of metal salts in step(c) is performed by adding the alkaline solution to the solution ofmetal salts or by adding the solution of metal salts to the alkalinesolution. Advantageously, the alkaline solution is added to the solutionof metal salts.

The alkaline solution (or the solution of metal salts) is added in sucha way that the pH of the solution formed in said step is maintained at aconstant value. More advantageously, the speed of adding the alkalinesolution to the solution of metal salts (or the solution of metal saltsto the alkaline solution) is set so as to maintain the pH of thesolution formed in said step constant (i.e. at a constant value). Theadding can be performed dropwise. Advantageously, the speed of adding is5 to 10 mL/min per liter solution of metal salts (or 5 to 10 mL/min perliter of alkaline solution when adding the solution of metal salts tothe alkaline solution).

Advantageously, the pH of the formed solution in the co-precipitationstep is comprised between 9 and 13, advantageously between 10 and 12,advantageously between 10 and 11.

More advantageously, in the co-precipitation step, the speed of addingthe alkaline solution to the solution of metal salts (or the solution ofmetal salts to the alkaline solution) is 5 to 10 mL/min per litersolution of metal salts (or per liter of alkaline solution) so as tomaintain the pH of the formed solution (by co-precipitation) at aconstant value comprised between 9 and 13, advantageously between 10 and12, advantageously between 10 and 11.

Advantageously, the co-precipitation step is performed at a temperatureup to 65° C., advantageously up to 50° C., advantageously up to 40° C.,advantageously up to 30° C. The co-precipitation step is advantageouslyperformed at a temperature comprised between 1° C. and 65° C.;advantageously between room temperature and 65° C., or alternativelybetween 1° C. and 50° C., advantageously between 1° C. and 40° C.,advantageously between 1° C. and 30° C., advantageously between 1° C.and room temperature (20° C.).

Methods of the present disclosure are performed in the absence ofcarbonates (e.g. Na₂CO₃). More particularly, in methods of the presentdisclosure, the co-precipitation is performed in alkaline environment(i.e. by an alkaline co-precipitation) in the absence of carbonates.According to the present disclosure the co-precipitation is performed inammonia solution.

After performing the co-precipitation step, the formed precipitate isaged. More particularly, during the aging step, the precipitate andsolution formed in step (c) are kept in contact with each other for aperiod of time. Advantageously, the step of aging the precipitate (step(d)) is performed for a period of at least 1 hour, advantageously for aperiod between 1 hour and 24 hours, advantageously from 6 hours to 24hours. The step d) of aging the precipitate is advantageously performedat a temperature comprised between 10° C. and 150° C., advantageouslybetween 10° C. and 65° C. Alternatively, the step d) of aging theprecipitate is advantageously performed at a temperature up to 65° C.(e.g. between 1° C. and 65° C.), advantageously up to 50° C.,advantageously up to 40° C.

Advantageously, step (d) is performed at a temperature being comprisedbetween 10° C. and 65° C., advantageously between room temperature and65° C., advantageously at 65° C., advantageously for a period of 1 hourto 24 hours. Alternatively, step (d) is performed by hydrothermaltreatment at a temperature being comprised between 80° C. and 150° C.,advantageously at 110° C., such as for a period of 1 hour to 24 hours.Advantageously, the hydrothermal treatment is performed in an autoclave.

After performing the aging step, the Ln-doped LDH precipitate isseparated from the solution. Advantageously, the precipitate comprising(or consisting of) Ln-doped LDH material is separated from the solutionby filtration or centrifugation.

Advantageously, after the separation step, the precipitate is washed(one or more times, with distilled water), and optionally dried toobtain a powdered Ln-doped LDH material. The (optional) drying of theprecipitate can be performed at a temperature comprised between 10° C.and 150° C., advantageously between room temperature and 80° C., for 1to 48 hours. More advantageously, the drying of the precipitate isperformed between room temperature and 60° C., for 6 to 24 hours.

Advantageously, methods according to the present disclosure can compriseone or a combination of the following aspects:

-   -   the alkaline solution comprises a 25% (w/w) ammonia solution;    -   the solution of metal salts consists of a 1M solution of        Mg(NO₃)₂.6H₂O (magnesium nitrate hexahydrate), Al(NO₃)₃.9H₂O        (aluminium nitrate nonahydrate) and La(NO₃)₃.6H₂O        (lanthanum(III) nitrate hexahydrate), the molar ratio of        Mg/Al/La being 3/0.9 to 0.95/0.05 to 0.1;    -   step (c) is performed at room temperature at a pH comprised        between 10 and 12;    -   step (d) is performed, e.g. for a period of 1 hour to 24 hours,        at a temperature being comprised between 10° C. and 150° C.        (advantageously at 65° C. for about 24 hours).

Advantageously, after step (d) (the step of aging), the precipitatecomprising (or consisting of) La-doped LDH material is separated fromthe solution by filtration or centrifugation. After the separation step,the precipitate is washed (one or more times, with distilled water) andoptionally dried to obtain a powdered La-doped LDH material. The dryingof the precipitate can be performed at 60° C., for 24 hours.

Advantageously, by performing a method of the present disclosure, thelattice parameter a₁₁₀ of the unit cell of the crystal structure of theformed (powdered) Ln-doped LDH material (advantageously La-doped LDHmaterial) is increased with at least 1.6% compared to the latticeparameter a₁₁₀ of the unit cell of the crystal structure of the non-Lndoped powdered LDH material (i.e. of a same LDH material which is notLn-doped). The increased lattice parameter a₁₁₀ confirms that thelanthanide cations (Ln³⁺) (advantageously lanthanum cations, La³⁺) areindeed incorporated in the lattice layers of the LDH material.

According to other aspects, the present disclosure is related to the useof lanthanide doped layered double hydroxides, advantageously lanthanum,europium or terbium doped layered double hydroxides, obtainable by amethod according to the present disclosure, as sorbent (or absorbent) oras catalyst. Advantageously, the Ln-doped LDHs (advantageously La-dopedLDHs) are used as sorbent for anions, such as organic anions orinorganic anions, in particular heavy metal anions, advantageously assorbent for heavy metal oxyanions.

In the context of the present application, the term oxyanion refers toan ion with generic formula A_(x)O_(y) ^(z−), where A represents achemical element and O represents an oxygen atom. Examples of oxyanionsare arsenite, arsenate, chromate, phosphate, selenite, selenate,vanadate, molibdate, manganate, borate, nitrate, etc.

The term (heavy) metal oxyanion refers to an ion with generic formulaA_(x)O_(y) ^(z−), where the element A represents a (heavy) metalchemical element and O represents an oxygen atom.

The Ln-doped LDHs (advantageously La-doped LDHs) obtainable by a methodaccording to the present disclosure can for example be used as sorbentin wastewater treatment (the wastewater comprising heavy metals, e.g.wastewater from hydrometallurgical processes) or in treatment ofalkaline leachates containing metal oxyanions. Alternatively, theLn-doped LDHs (or calcined forms thereof, advantageously La-doped LDHsor calcined forms thereof) can be used as catalyst in reactions in highpH environment such as photocatalysis, or as basic catalysts in forexample aldol condensation reactions. The Ln-doped LDHs (advantageouslyLa-doped LDHs) can be used as well in the treatment of waste of toxiceffluents generated in the electroplating industries, coal burningindustries, refining, insecticides, fungicides, and in iron and steelproducing industries.

Advantageously, the Ln-doped LDHs (advantageously La-doped LDHs)obtainable by a method according to the present disclosure areeco-friendly (non-toxic).

The Ln-doped LDHs (advantageously La-doped LDHs) obtainable by a methodaccording to the present disclosure can directly be applied in highalkaline streams, with no need of prior reduction of pH.

Advantageously, the Ln-doped LDHs (advantageously La-doped LDHs)obtainable by a method according to the present disclosure are used assorbent at a pH comprised between 7 and 14, advantageously between 8 and13, advantageously between 10 and 12, advantageously between 11 and 12(for example at pH of 11.5).

In further aspects, the present disclosure is related to lanthanidedoped layered double hydroxides (Ln-doped LDH) obtainable by a methodaccording to the present disclosure. More particularly, the presentdisclosure is related to lanthanide-doped layered double hydroxides (inparticular La-doped LDH) obtainable by a method according to the presentdisclosure, wherein the lattice parameter a₁₁₀ of the unit cell of thecrystal structure of the LDH material is increased with at least 1.6%compared to the lattice parameter a₁₁₀ of the unit cell of the crystalstructure of the non-La doped LDH material (i.e. of a same LDH materialwhich is not La-doped). The increased lattice parameter a₁₁₀ confirmsthat lanthanum (more particularly La³⁺ cations) is (are) indeedincorporated in the lattice layers of the LDH material.

The Ln-doped LDH obtainable by a method according to the presentdisclosure can be used as sorbent (advantageously as sorbent for heavymetal anions, more advantageously as sorbent for heavy metal oxyanions),or the Ln-doped LDHs (or calcined forms thereof) can be used ascatalyst. More particularly, the Ln-doped LDH can be used as sorbent ina high alkaline environment, i.e. at a pH comprised between 7 and 14,advantageously between 8 and 13, advantageously between 10 and 12,advantageously between 11 and 12 (for example at pH of 11.5).

Advantageously, the Ln-doped LDHs obtainable by a method according tothe present disclosure are eco-friendly (non-toxic).

EXAMPLES

The present disclosure is further illustrated by means of the followingexamples in which two types of layered double hydroxides (LDHs) havebeen considered. A first LDH type is the Ca²⁺ containing layeredmaterial, i.e. the group of hydrocalumites (throughout the presentdescription referred to as HC) in which the divalent calcium ions arehepta-coordinated. The layers consist of a combination of calciumhydroxide decahedra- and hexa-coordinated trivalent hydroxide octahedras(as for example described by Grover et al. in Applied Clay Science 2010,48(4), 631-637 and by Tian et al. in Journal of Chemistry 2014, 2014,8). Hydrocalumite has the general formula Ca₂Al(OH)₆Cl.2H₂O withhepta-coordinated Ca²⁺ and hexa-coordinated Al³⁺ units, respectively.

A second LDH type is the Mg²⁺ containing layered materials, i.e. thegroup of hydrotalcites (throughout the present description referred toas HT) in which the divalent magnesium ions are hexa-coordinated (as forexample described by Cavani et al. in Catalysis Today 1991, 11(2),173-301). The layers consist of a combination of magnesium and aluminiumions both hexa-coordinated in octahedras. Hydrotalcite-type material hasthe general formula Mg₆Al₂CO₃(OH)₁₆.4(H₂O).

Within the context of the present disclosure Hydrotalcite-type materialscomprise metal ions like Mg, Al and/or Zn, but also others like Ni, Fe,Cu, Co . . . , in which the metals are hexacoordinated in thebrucite-like sheets and include divalent metals hexacoordinated in theLDH sheets (such as Mg2+, Zn2+, Ni2+, Fe2+, Cu2+, etc. . . . );trivalent metals hexacoordinated in the LDH sheets (such as Al3+, Fe3+,Mn3+, etc. . . . ) or combinations of di- and trivalent metal cationsare also possible: multicomponent LDHs—example MgZnAlLn, MgCuAlLn,MgZnFeLn . . . or even containing tetravalent cations Me4+: Sn4+ or Ti4+(key cations for catalytic applications), example: MgAlSnLn, orMgAlTiLn, etc. . . . .

Advantageously, the lanthanide-doped HT-type LDH's comprise combinationsof di- and trivalent metal cations wherein the metal-cations arehexacoordinated in the LDH sheets with cationic ratios in the range ofMe2+/Me3+/Ln3+: 2-4/0.8-0.95/0.05-0.2; in particular Me2+/Me3+/Ln3+:4-5/0.95/0.05. In addition, or alternatively, the lanthanide-dopedHT-type LDH's comprise tetravalent metal cations with cationic ratios inthe range of Me2+/(Me3++Me4+)/Ln3+: 2-4/0.8-0.95/0.05-0.2.

In Example 1, the isomorphous substitution of Al³⁺ with Ln³⁺ cations inMgAl-hydrotalcite is compared with a comparative substitution inCaAl-hydrocalumites.

Furthermore, in Example 2, the adsorption of (oxy)anions by non-dopedand doped LDH-type materials is studied. Indeed, LDH-type materials canadsorb (oxy)anions (such as inorganic anions (e.g. nitrate, sulphate,chromate, etc.), and/or organic anions (e.g. acetate, tartrate,carboxylate, etc.)) via an anionic exchange mechanism where theinterlayer anions between two sheets (i.e. the interlayer anions in theinterlayer region between two neighbouring brucite-like sheets) arereplaced by new anions (e.g. by (oxy)anions present in the treatedwastewater). Sorption performance of the materials is thereby known tobe influenced by the type of material and the type of intercalatedanion. This phenomenon is controlled by the anionic affinity for LDHswhich is well known in the art (as for example described by Goh et al.in Water Res. 2008, 42(6-7), 1343-1368; or by Xu et al. in Handbook ofLayered Materials, S. M. Auerbach, K. A. Carrado, P. K. Dutta (Eds.),Marcel Dekker Inc., 2004, p. 373), to be in the following order:

NO³⁻<Br⁻<Cl⁻<F⁻<OH⁻<SO₄ ²⁻<CrO₄ ²⁻<HAsO₄ ²⁻<HPO₄ ²⁻<CO₃ ²⁻ withpreferential affinity for the anions with higher valence rather than formonovalent state. From this order, it can for example be derived thatnitrate anions (NO₃ ⁻) can easily be exchanged with chromate anions(CrO₄ ²⁻).

Example 1: Isomorphous Substitution of Al³⁺ by La³⁺ in CaAl-HC Versus inMgAl-HT

In this example, La-doped HC- is synthesised using a comparative methodand HT-type material is synthesised using a method of the presentdisclosure. The crystalline structure of the obtained LDH material isthen investigated via X-ray diffraction analysis (XRD). Furthermore,images of the morphology of the LDH material are produced using ascanning electron microscope (SEM). The obtained results areadditionally compared to the characteristics of the HC- and HT-typematerial obtained by performing the same method except for performingthe step of doping with La.

1.A. Comparative Example: Non-La Doped HC-Type Material

Non-La doped HC-type material is synthesised by slowly adding (bydropwise adding, with speed of 4.16 mL/min for 120 minutes) basic sodiumhydroxide solution (NaOH 2M) as co-precipitation agent to 500 mL of amixed solution (1M in total) of metal precursor salts. The sodiumhydroxide solution does not comprise carbonates (e.g. Na₂CO₃). Thesolution of metal salts contains CaCl₂.2H₂O and AlCl₃.6H₂O, and does notcomprise a metal salt of a lanthanide (hence, does not comprise a metalsalt of lanthanum either). Furthermore, the solution of metal salts doesnot comprise carbonates either. The solution of metal salts is stirredto obtain the mixed (or stirred) solution, before performing the step ofadding the sodium hydroxide solution to the solution of metal salts. Bythen slowly adding the sodium hydroxide solution to the (mixed) solutionof metal salts, non-Ln doped, more particularly non-La doped, LDHprecipitate in solution is formed by co-precipitation at roomtemperature while the pH of the solution is maintained at a constantvalue of 11.5. The formed precipitate is then aged at 65° C. for 24 h,separated by filtration, washed with distilled water and dried at 60° C.for 24 hours to obtain powdered non-La doped LDH material. Moreparticularly, non-La doped hydrocalumite-type Ca2Al material is obtainedwith Cl⁻ as interlayer anions.

Table 1A reports the synthesis conditions and structural characteristicsof the formed non-La doped hydrocalumite-type Ca2Al material (withcationic ratio Ca/Al being 2/1).

TABLE 1A non-La doped Ca2Al HC-type structure cationic Co-precip- unitcell ratio La itation aging parameters of Ca/Al (mol %) agent pH T LDHphase 2 to 4/1 0 NaOH 2M 11.5-12 65° C. 2θ = 11.302°- 11.34° d₀₀₂ =7.823- 7.790 Å c₀₀₂ = 15.645- 15.593 Å 2θ = 31.14° d₁₁₀ = 2.872 Å a₁₁₀ =5.745 Å

The structure and morphology of the Ca2Al material is confirmed by XRDand SEM, respectively, as can be seen from FIG. 3 showing the XRDpattern (left) and SEM image (right) of the obtained non-La doped Ca2AlHC-type structure. The unit cell parameters of the LDH phase are givenin Table 1A.

Moreover, by increasing the calcium content (cationic ratio Ca/AI being3/1 and 4/1), Ca3Al-LDH and Ca4Al-LDH are successfully synthesised aswell, the structure of which is confirmed by XRD as can be seen fromFIG. 4 showing the XRD patterns of the obtained non-La doped HC-typematerials (a) Ca2Al, (b) Ca3Al and (c) Ca4Al.

1.B. Comparative Example: La-Doped HC-Type Material

La-doped HC-type material is synthesised by by slowly adding (bydropwise adding, with speed of 4.16 mL/min for 120 minutes) basic sodiumhydroxide solution (NaOH 2M) as co-precipitation agent to 500 mL of amixed solution (1M in total) of metal precursor salts. The sodiumhydroxide solution does not comprise carbonates (e.g. Na₂CO₃). In thepresent example, the solution of metal salts contains CaCl₂.2H₂O,AlCl₃.6H₂O, and LaCl₃.7H₂O. The solution of metal salts does notcomprise carbonates either. The solution of metal salts is stirred toobtain the mixed (or stirred) solution, before performing the step ofadding the sodium hydroxide solution to the solution of metal salts. Bythen slowly adding the sodium hydroxide solution to the (mixed) solutionof metal salts, La-doped LDH precipitate in solution is formed byco-precipitation at room temperature while the pH of the solution ismaintained at a constant value of 11.5. The formed precipitate is thenaged at 65° C. for 24 h, separated by filtration, washed with distilledwater and dried at 60° C. for 24 hours to obtain powdered La-doped LDHmaterial. More particularly, La-doped hydrocalumite-type CaAlLa materialis obtained with Cl⁻ as interlayer anions.

Table 1B reports the synthesis conditions and structural characteristicsof the formed La-doped hydrocalumite-type CaAlLa material.

TABLE 1B La-doped CaAlLa HC-type structure cationic Co-precip- unit cellratio La itation aging parameters of Ca/Al/La (mol %) agent pH T LDHphase 2/0.5 to 0.9/ 3.3 to NaOH 2M 11.5-12 65° C. 2θ = 11.325°- 0.1 to0.5 16.5 11.345° d₀₀₂ = 7.85- 7.82 Å c₀₀₂ = 15.71- 15.64 Å 2θ = 31.14-31.15° d₁₁₀ = 2.86- 2.875 Å a₁₁₀ = 5.73- 5.75 Å

The structure of the LDH material is confirmed by XRD, the unit cellparameters of the LDH phase given in Table 1B.

FIG. 5 shows the obtained XRD patterns of (a) Ca2Al (non-La doped); (b)Ca2Al0.9La0.1 (3.3 m % La), (c) Ca2Al0.8La0.2 (6.6 m % La), (d)Ca2Al0.5La0.5 (16.5 m % La) and (e) La(OH)₃ reference material.

The XRD characterization indicates the formation of a partialsegregation phase of La(OH)₃ or La₂O₃ with increase of substitutiondegree. In other words, the crystallinity of the LDH material decreaseswith increased degree of La³⁺ doping.

The Ca2Al0.9La0.1 sample with 3.3 m % La-doping shows the lowest degreeof segregated La-phases (La(OH)₃ or La₂O₃) but also has a lowcrystallization degree of the LDH phase. A further optimization of thecrystallization degree of the Ca2Al0.9La0.1 (3.3 m % La) LDH phase isperformed by optimizing the synthesis conditions, i.e. by performing theco-precipitation at RT followed by aging (a) at RT, (b) at 65° C., (c)via hydrothermal treatment in an autoclave at 110° C. and (d) La(OH)₃reference material, as shown in FIG. 6.

Furthermore, from Table 1A (non-La doped CaAl HC-type structure) andTable 1B (La-doped CaAlLa HC-type structure), i.e. comparing thenon-doped HC-type structure with the doped HC-type structure, it can beseen that the unit cell parameter a₁₁₀ increases with the amount of La³⁺content. This confirms that La³⁺ is indeed incorporated in the latticelayers of the HC-type structure of Example 1.B.

1.C. Comparative Example: Non-La Doped HT-Type Material

Non-La doped HT-type material is synthesised by slowly adding (bydropwise adding, with speed of 4.16 mL/min for 120 minutes) basicammonia solution (25 wt % NH₃) as co-precipitation agent to 500 mL of amixed solution (1M in total) of metal precursor salts. The ammoniasolution does not comprise carbonates (e.g. Na₂CO₃). The solution ofmetal salts contains Mg(NO₃)₂.6H₂O and Al(NO₃)₃.9H₂O, and does notcomprise a metal salt of a lanthanide (hence, does not comprise a metalsalt of lanthanum either). Furthermore, the solution of metal salts doesnot comprise carbonates either. The solution of metal salts is stirredto obtain the mixed (or stirred) solution, before performing the step ofadding the ammonia solution to the solution of metal salts. By thenslowly adding the ammonia solution to the (mixed) solution of metalsalts, non-Ln doped, more particularly non-La doped, LDH precipitate insolution is formed by co-precipitation at room temperature while the pHof the solution is maintained at a constant value of 10. The formedprecipitate is then aged at 65° C. for 24 h, separated by filtration,washed with distilled water and dried at 60° C. for 24 hours to obtainpowdered non-La doped LDH material. More particularly, non-La dopedhydrotalcite-type MgAl material is obtained with NO₃ ⁻ as interlayeranions.

Table 1C reports the synthesis conditions and structural characteristicsof the formed non-La doped hydrotalcite-type MgAl material.

TABLE 1C non-La doped MgAl HT-type structure cationic co-precip- unitcell ratio La itation parameters of Mg/Al (mol %) agent pH aging T LDHphase 3/1 0 ammonia 10 65° C. 2θ = 9.98° solution d₀₀₃ = 8.856 Å (25%(w/w) c₀₀₃ = 26.568 Å NH₃) 2θ = 60.98° d₁₁₀ = 1.5482 Å a₁₁₀ = 3.032 Å

The structure of the LDH material is confirmed by XRD. FIG. 7 shows theXRD pattern of the obtained Mg3Al LDH phase (curve (a), non-La dopedHT-type material). The unit cell parameters of the LDH phase given inTable 1C.

1.D. Example: La-Doped HT-Type Material

La-doped HT-type material is synthesised by slowly adding (by dropwiseadding, with speed of 4.16 mL/min for 120 minutes) basic ammoniasolution (25 wt % NH₃) as co-precipitation agent to 500 mL of a mixedsolution (1M in total) of metal precursor salts. In the present example,an ammonia solution is used, instead of NaOH, as co-precipitating agent.The ammonia solution does not comprise carbonates (e.g. Na₂CO₃). Thesolution of metal salts contains Mg(NO₃)₂.6H₂O, Al(NO₃)₃.9H₂O andLa(NO₃)₃.6H₂O. The solution of metal salts does not comprise carbonateseither. The solution of metal salts is stirred to obtain the mixed (orstirred) solution, before performing the step of adding the ammoniasolution to the solution of metal salts. By then slowly adding theammonia solution to the (mixed) solution of metal salts, Ln-doped, moreparticularly La-doped, LDH precipitate in solution is formed byco-precipitation at room temperature while the pH of the solution ismaintained at a constant value of 10. The formed precipitate is thenaged at 65° C. for 24 h, separated by filtration, washed with distilledwater and dried at 60° C. for 24 hours to obtain powdered La-doped LDHmaterial. More particularly, La-doped hydrotalcite-type MgAlLa materialis obtained with NO₃ ⁻ as interlayer anions.

Table 1D reports the synthesis conditions and structural characteristicsof the formed La-doped hydrotalcite-type MgAlLa material.

TABLE 1D La-doped MgAlLa HT-type structure cationic co-precip- ratio Laitation unit cell Mg/Al/La (mol %) agent pH aging T parameters 3/0.9 to1.65 to ammonia 11.5-12 65° C. 2θ = 10.7°- 0.95/ 3.3 solution 10.9° 0.05to (25% d₀₀₃ = 8.1103- 0.1 (w/w) 8.3236 Å NH₃) c₀₀₃ = 24.33- 24.97 Å 2θ= 60.05- 60.31° d₁₁₀ = 1.523- 1.525 Å a₁₁₀ = 3.067- 3.08 Å

The structure of the LDH material is confirmed by XRD. FIG. 7 shows theXRD patterns of (a) non-La doped Mg3Al of Example 1.C; and of (b)Mg3Al0.95La0.05 (1.65 m % La) and (c) Mg3Al0.9La0.1 (3.3 m % La)(La-doped HT-type material of the present Example 1.D). The unit cellparameters of the LDH phase given in Table 1 D.

The XRD characterization in FIG. 7 indicates the synthesis ofLa-substituted Mg3Al-LDH without La³⁺ segregation for both 1.65 m % and3.3 m % La³⁺ content. The ammonia co-precipitation hence favours thedoping of the structure with lanthanum (via isomorphous substitution) inthe brucite-like layers when compared with the co-precipitationperformed in NaOH (cf. Example 1.B, forming La-doped HC-type material).In other words, the co-precipitation is performed in ammonia solution,thereby forming La-doped HT-type material. More advantageously, La-dopedHT-type material is formed with an interlayer of nitrate anions.

Furthermore, from Table 1C (non-La doped MgAl HT-type structure) andTable 1D (La-doped MgAlLa HT-type structure), i.e. comparing thenon-doped HT-type structure with the doped HT-type structure, it can beseen that the unit cell parameter a₁₁₀ increases with the amount of Lacontent. This confirms that La³⁺ is indeed incorporated in the latticelayers of the HT-type structure of Example 1.D using a method of thepresent disclosure (the Shannon ionic radius of La (1.03 Å) being higherthan the ionic radius of Al (0.54 Å) and of Mg (0.74 Å)). The doping ofthe structure with lanthanum is thereby performed (via isomorphoussubstitution) in the brucite-like layers (and not in the interlayerregion between two neighbouring brucite-like sheets).

Example 2: Sorption Performance

In this example, the adsorption of (oxy)anions by LDH-type materials isstudied. More particularly, the sorption capacity is measured for non-Ladoped and La-doped hydrotalcite-type materials.

In the present example, the chromate uptake by non-La doped and La-dopedhydrotalcite-type materials from solutions with pH from 8 up to 13 hasbeen tested. The tested LDH materials have different intercalatedanions: NO₃ ⁻, CO₃ ²⁻, and Cl⁻. The results are compared with PURAL® MG63 HT, a commercial available hydrotalcite material provided by SasolPerformance Chemicals.

FIG. 8 shows the adsorbed chromate amounts (adsorption capacity atequilibrium q_(e), in mgCr⁶⁺/g) at pH 8 as a function of time forconventional non-La doped hydrotalcite-type (LDH) materials havingdifferent intercalated (i.e. the exchangeable) anions: ZnAl-type (left)and MgAl-type (right), compared with PURAL® MG 63 HT.

Furthermore, the sorption efficiency of chromate (CrO₄ ²⁻, with Cr⁶⁺)and material stability during sorption tests at different pH forconventional non-La doped hydrotalcite-type (LDH) materials have beenevaluated in FIG. 9 with ZnAl-type hydrotalcites (left) and MgAl-typehydrotalcites (right). As can be seen from FIG. 9, the chromate uptakeis strongly dependent on the pH: it decreases with increase of pH (from8 to 13) due to low material stability at such high pH, and, withoutbeing bound to theory, competition with OH⁻ anions present in the LDHmaterial, strongly limiting the sorption capacity of these materials.Indeed, increasing the pH greatly affects the sorption performances: itis observed from FIG. 9, that the ZnAl-type material is less stable thanthe MgAl-type material, which is in accordance with the cationspeciation as a function of the pH as depicted in the Pourbaix diagrams(or E_(h)—pH diagram) in FIG. 10.

From FIG. 9, it hence follows that non-doped LDH materials can easily beapplied for chromate sorption in aqueous streams with a pH ranging from7 to 10. However, due to their strong material instability in higheralkaline conditions (pH ranging from 10 to 13), it is clear that theyare not suited at all to be used as sorbent in this higher range of pH.

Table 2 further summarizes sorption performances measured for non-dopedCaAl-type hydrocalumites and MgAl-type hydrotalcites, as well as forLa-doped CaAl-type HC and for MgAl-type HT doped with La according to amethod of the present disclosure.

TABLE 2 Sorption performances for non-doped and La-doped CaAl HC-typeand MgAl HT-type structures q_(e) mgCr⁶⁺/g efficiency Mg²⁺ Ca²⁺ Al³⁺La³⁺ Sample cationic ratio pH (max. 20 min) % (ppm) (ppm) (ppm) (ppm)CaAl 2/1 8 5 18.21 — 134 52.3 — 13 2 7.4 — 26.9 21.9 — CaAlLa 2/0.9/0.18 6 22.22 — 147 49.8 0 13 7 25.9 — 29.8 10.8 0 CaAlLa 3/0.9/0.1 8 414.81 — 90 47.3 0 13 0 0 — 2.53 11.9 0 CaAlLa 4/0.9/0.1 8 4 14.81 — 10338.5 0 13 0.2 2.86 — 26.3 11.6 0 MgAl 3/1 8 29 90.63 4.22 — 0 — 13 932.14 0 — 6.63 — MgAlLa 3/0.95/0.5 8 25.8 99.37 19.30 — 0.59 0 13 1037.04 0 — 0.856 0 MgAlLa 3/0.9/0.1 8 25.8 95.55 7.35 — 0 0 13 10 37.04 0— 0.622 0

From Table 2 it is observed that sorption performance is increased forthe MgAl HT-type materials when compared to the CaAl HC-type materials.Furthermore, at pH 13 the sorption performance for the MgAl HT-typematerial increases from 32% to 37% with insertion of La³⁺ cations (i.e.for La-doped MgAl HT-type material formed using a method of the presentdisclosure). At pH 13 extremely low leaching of metal elements from theLDH structure is observed for the La-doped MgAl HT-type materials, i.e.lower than 1.5 ppm per gram material per hour at pH 13.

FIG. 11 illustrates, for the MgAl HT-type material listed in Table 2,(left) the sorption efficiency during sorption tests at different pH and(right) measurements of metal evolution during sorption tests (i.e.metals leaching from the sorbent material after 20 minutes of sorptionexperiment, being related with material stability) as a function of thepH.

Moreover, it is confirmed by XRD structural characterization, as shownin FIG. 12, that the structure of the La-doped MgAl HT-type material ispreserved after the sorption tests at pH 8 and pH 13, the anion exchangebeing proved by the change in the interlayer spacing (indicated by theunit cell parameter which gives indication on the size of theintercalated anions).

From the above examples, it follows that the La-doped LDHs obtained by amethod of the present disclosure are stable in high alkaline media. Moreparticularly, the sorption capacity of the La-doped Mg2+ containing LDHsobtained by a method of the present disclosure is higher compared to thestate-of-the-art sorbents when used in high alkaline conditions. Indeed,the La-doped LDH sorbent shows sorption capacity at pH 13 of 10 mg/gsorbent or more (even 15 mg/g up to 30 mg/g sorbent). Therefore, theycan be applied as sorbent material directly in high alkaline streams(with pH ranging from 10 to 13.5), with no need of prior reduction ofpH. The costly operation of first reducing the pH to a more acidic range(as needed using sorbents known in the art up to now) can hence beavoided.

Even though in the above examples Mg²⁺ containing layered HT-materialdoped with lanthanide (more particularly lanthanum) have been producedand tested, it will be clear to those skilled in the art that also othertypes of LDH material doped with lanthanide (more particularlylanthanum) can be produced by the method of the present disclosure, suchas Zn²⁺ containing layered HT-material (zaccagnaite, Zn-LDH) doped withlanthanide (more particularly lanthanum).

Example 3: Testing Different Reaction Conditions During theCo-Precipitation Step

In this experiment an La-doped HT-material is synthesized in accordancewith the experiment described in example 1.D, i.e. using the samereagents but whereas in 1.D the co-precipitation step is performed atroom temperature, and a pH of 10, now different pH's and differenttemperatures during the co-precipitation step were tested.

The results are summarized in Table 1 below and indicate that for a pHin the range of 10-11 there is no influence of the temperature on theformation of La-doped HT-material. At a higher pH and temperature,La(OH)₃ starts to segregate from the material. It follows from thisexperiment that for the La-doped HT-material the pH is preferablybetween 10 and 11 irrespective of the temperature at which theco-precipitation step is being performed.

TABLE 3 Synthesis conditions and structural characteristics of theLa-doped hydrotalcite- type MgAlLa material at different pH andtemperatures of co-precipitation step. pH of co- T(° C.) of co- precip-precip- Phase Unit cell La itation itation observed parameters ofCationic ratio (mol %) step step by XRD LDH phase Mg3Al0.95La0.05 1.6510-12 1 LDH a₁₁₀ = 3.08-3.09 Å c₀₀₃ = 26.27-26.47 Å Mg3Al0.95La0.05 1.6510-12 15 LDH a₁₁₀ = 3.08-3.09 Å c₀₀₃ = 26.21-26.43 Å Mg3Al0.95La0.051.65 10-11 30 LDH a₁₁₀ = 3.08-3.10 Å c₀₀₃ = 25.78-26.35 Å

Per reference to FIG. 13, when increasing the temperature during theco-precipitation step larger volumes of the ammonia solution arerequired to keep the pH at the desired amount (e. g. pH 11-12, morespecifically pH 12, at 30° C.). This is due to the endothermic nature ofthe co-precipitation process which leads to a temperature decrease forthe LDH phase formation. Therefore, the increase of temperature willshift the reaction equilibrium towards the initial products and as aconsequence, more ammonia has to be added during synthesis for the LDHphase formation.

Thus notwithstanding the fact that the temperature has no direct impacton the La-doped LDH being formed, it is beneficial to perform theco-precipitation reaction at lower temperatures such as up to 15° C. (atthe tested temperatures 1° C., 4° or 15° only small volumes of ammoniasolution are needed to keep the co-precipitation step at the desired pH)for the economics of the reagents to be used.

Example 4: Testing Different Reaction Conditions During the Aging Step

In this experiment an La-doped HT-material is synthesized in accordancewith the experiment described in example 1.D, i.e. using the samereagents but whereas in 1.D the co-precipitation step is performed atroom temperature, and a pH of 10 with an aging of the precipitate at 65°C. for 24 h. The co-precipitation was now done at a pH of 11, and wetested different temperatures for the aging step.

The results are summarized in Table 4 and indicate that even around roomtemperature a high degree of crystallinity is obtained, withoutsegregation.

TABLE 4 Synthesis conditions and structural characteristics of theLa-doped hydrotalcite-type MgAlLa material at different temperatures ofaging step. La pH of T(° C.) Phase Unit cell (mol aging of observedparameters of Cationic ratio %) step aging by XRD LDH phaseMg3Al0.95La0.05 1.65 11 15 LDH a₁₁₀ = 3.09 Å c₀₀₃ = 26.37 ÅMg3Al0.95La0.05 1.65 11 30 LDH a₁₁₀ = 3.08 Å c₀₀₃ = 26.41 Å

Example 5: Testing Different Lanthanides

To test the feasibility of synthesizing other lanthanides comprisingLDHs wherein the lanthanide is doped into the brucite-like layer of thelayered double hydroxides without segregation, other lanthanides weretested using the protocol as described in example 1.D. Theco-precipitation step being performed at pH 11 and at 15° C. and theaging step at 30° C.

The crystalline structure of the obtained LDH material is theninvestigated via X-ray diffraction analysis (XRD) and shown in FIG. 14.It is evident, and when compared to the X-Ray diffraction of theLanthanum-doped material (a), that the crystalline structure of alltested lanthanides is effectively the same.

In addition, the materials doped with europium (Eu) or terbium (Tb) havefluorescent properties as evident from the Raman spectra shown for Eu inFIG. 15.

Referring to FIG. 15, Raman measurements were done using a HORIBA XploRAPLUS V1.2 MULTILINE Cofocal Raman microscope under the same settingconditions. A 532 nm diode-pumped solid-state (DPSS) laser with a powerof 25 mW was used for excitation and all the spectra were collected withan accumulation time of 10 s.

The low spectra range (below 1200 cm⁻¹) is enlarged for better claritydue to the high intensity of the fluorescence bands above 1500 cm⁻¹. TheRaman spectra above 1500 cm⁻¹ comprises the total luminescence ofEuropium. Detailed assignment of Raman bands is included in Table 5.

TABLE 5 Assignment of Raman bands of FIG. 15 Raman shift, cm⁻¹Assignment Reference 493 EgT vibration mode of M-OH in LDH 1, 2 lattice572 EuT vibration mode of M-OH in LDH 1, 2 lattice 740 EuR(OH) vibrationmode 1, 2 1055 Vibration mode of nitrate anions in the 1, 2 interlayer(fingerprint) 1530 Eu³⁺ transition 3, 4 1801, 1930, 2020 Characteristicsplit of luminescence 3, 4 bands assigned to Eu³⁺ (fingerprint) 2553,2585 Split of electronic transition ⁵D₀-⁷F₁ of 4 Eu³⁺ 3450 Electronictransition ⁵D₀-⁷F₂ of Eu³⁺ 4 References: 1. E. M. Seftel et. al, AppliedClay Science 165 (2018) 234-246. 2. S. S. C. Pushparaj et. al, J. Phys.Chem. C 119 (49) (2015) 27695-27707. 3. Tao Wu et al, Anal. Chem. 88(2016) 8878-8885. 4. C. Tiseanu et. al, Phys. Chem. Chem. Phys. 14(2012) 12970-12981.

Example 6: Synthesis of Methyl Octyl Carbonate—Catalyst Testing

Besides the commercial hydrotalcite catalytic material, the catalyticactivity of the lantanide doped materials obtained using the method ofthe present disclosure, hereinafter referred to as MK14 and MK51, weretested in a catalytic reaction. The synthesis route for the lanthanidedoped materials are summarized in Table 6.

Firstly, three samples of catalyst-hydrotalcite (MK8-14, MK51-35C andcommercial hydrotalcite (called H550—Sigma_Aldrich—CAS Number11097-59-9) were tested catalytically, pretreated under calcination at550° C. for 5 hours. After that, the catalysis with strong acids (ashomogeneous catalysts for comparison) (Toluenesulfonic acid (TSO₃H) andH₂SO₄)) was carried out according to the following protocol; wherein thechemicals and the amounts used, calculations, for each of the testedcatalysts are indicated in Tables 7.1-7.5. below:

A g of 1-Octanol and B mL of Dimethyl carbonate (DMC) were mixed in a 50mL round bottom flask. When a good mixture of the reactants was reached,x gram of hydrotalcite (10% mass of 1-Octanol) and acids were added,respectively.

The reaction was carried out for 22 hours at 90° C. During the reaction,samples were taken out for GC-MS analysis after 2 h, 4 h, 5.5 h and 22h.

The catalytic results are given in FIG. 16 after analysis with GC-MSafter 2 h, 4 h, 5.5 h and 22 h. The area percentages of the detectedpeaks were calculated for 1-octanol and the reaction products Methyloctyl carbonate and Dioctyl carbonate, respectively and plotted (graphx).

TABLE 6 Synthesis route for MK8_14 and MK51-35C (Aging time: 24 hours;RT: room temperature) salt solution amount Aging amount of sample volumeprecipitant synthesis Temp pH after drying powder name sampledescription (ml) precipitant (ml) temp (° C.) pH synthesis temp (g)MK8_14 Mg3Al0.95La0.05—NO3 100 Ammonia 72 RT RT 11.00 10.4 60 7.49 pH 1128-30% MK 51- Mg3Al0.95La0.05—NO3 100 Ammonia 200 RT 65 11.00 10.26 607.64 35C 25%

TABLE 7.1 Chemicals and amounts used, calculations (catalyst is MK8-14)Density MK8_14 M (g/mol) m (g) d ml n (mol) Equivalent DATA 1-Octanol130.23000 10.06390 (A) 0.07728 1.00000 GC MS: DMC 90.08000 34.80596 (B)1.07000 0.38639 5.00000 2 h, 4 h, Hydrotalcite  1.00639 (x) 10% mass 5.5h, 22 h

TABLE 7.2 Chemicals and amounts used, calculations (catalyst isMK51-3SC) density MK51-35C M (g/mol) m (g) d ml n (mol) Equivalent DATA1-Octanol 130.23000 10.13190 (A) 0.07780 1.00000 GC MS: DMC 90.0800033.92350 (B) 1.07000 0.38900 5.00000 2 h, 4 h, Hydrotalcite  1.05040 (x)10% mass 5.5 h, 22 h

TABLE 7.3 Chemicals and amounts used, calculations (catalyst ishydrotalcite H550) Commercial Density yield H550 M (g/mol) m (g) d ml n(mol) Equivalent (%) DATA 1-Octanol 130.23000 10.10390 (A) 0.077591.00000 GC MS: DMC 90.08000 34.21220 (B) 1.07000 0.38793 5.00000 2 h, 4h, Hydrotalcite  1.06520 (x) 10% mass 5.5 h, 22 h

TABLE 7.4 Chemicals and amounts used, calculations (catalyst istoluenesulfonic acid) Toluenesulfonic Density acid M (g/mol) m (g) d mln (mol) Equivalent DATA 1-Octanol 130.23000 10.05950 (A) 0.07724 1.00000GC MS: DMC 90.08000 33.69790 (B) 0.38622 5.00000 2 h, 4 h,Toluenesulfonic 190.20000  0.76250 (x) 1.24000 0.61492 0.00386 0.050005.5 h, 22 h acid monohydrate

TABLE 7.5 Chemicals and amounts used, calculations (catalyst is H₂SO₄)Density n (mol) H₂SO₄ M (g/mol) m (g) d ml (1H+) Equivalent DATA1-Octanol 130.23000 10.10370 (A) 0.07758 1.00000 GC MS: DMC 90.0800034.33300 (B) 0.38792 5.00000 2 h, 4 h, H₂SO₄ (98%) 98.07900  0.19023 (x)1.84000 0.10000 0.00388 0.05000 5.5 h, 22 h

1. A method for producing lanthanide doped layered double hydroxides(Ln-doped LDHs), the method comprising: a) preparing a carbonate freealkaline solution; b) preparing a solution of metal salts comprising asalt of a lanthanide; c) adding the alkaline solution and the solutionof metal salts to form a mixture, wherein a pH of the mixture ismaintained at a constant value so as to form Ln-doped LDH precipitate;d) aging the precipitate; and e) separating the precipitate from themixture; wherein the carbonate free alkaline solution is an aqueousammonia solution.
 2. The method of claim 1, wherein the aqueous ammoniasolution has a NH₃ concentration between 20% w/w and 30% w/w.
 3. Themethod of claim 1, wherein in step (c) adding the alkaline solution andthe solution of metal salts is performed by adding the alkaline solutionto the solution of metal salts, a speed of adding being 5 to 10 mL/minper liter solution of metal salts.
 4. The method of claim 1, wherein instep (c) the pH of the formed mixture is comprised between 9 and
 13. 5.The method of claim 1, wherein step (c) is performed at a temperaturecomprised between 1° C. and 65° C.
 6. The method of claim 1, wherein thesolution of metal salts comprises a salt of a lanthanide, and salts ofone or more of divalent, trivalent, and tetravalent metal cations. 7.The method of claim 6 wherein in the solution of metal salts thecationic ratios of Me2+/(Me3+ and/or Me4+)/Ln3+ are2-4/0.8-0.95/0.05-0.2, wherein Me refers to a metal element.
 8. Themethod of claim 6, wherein the solution of metal salts comprises a saltof a lanthanide, aluminium and one or more of calcium, magnesium, andzinc.
 9. The method of claim 8, wherein in the solution of metal saltsthe molar ratio of Ca/Al/Ln is 2 to 4/0.5 to 0.95/0.05 to 0.5, or themolar ratio of Mg/Al/Ln is 2 to 4/0.5 to 0.95/0.05 to 0.5, or the molarratio of Zn/Al/Ln is 2 to 4/0.5 to 0.95/0.05 to 0.5.
 10. The method ofclaim 8, wherein the solution of metal salts comprises a salt of alanthanide, aluminium, and magnesium.
 11. The method of claim 10,wherein in the solution of metal salts the molar ratio between Mg, Aland Ln is Mg_(x)Al_(y)Ln_(z), wherein x is between 2 and 4, y is between0.9 and 0.95, and z is between 0.05 and 0.1.
 12. The method of claim 1,wherein the solution of metal salts comprises: CaCl₂.2H₂O, AlCl₃.6H₂Oand LaCl₃.7H₂O; or Mg(NO₃)₂.6H₂O, Al(NO₃)₃.9H₂O and La(NO₃)₃.6H₂O; orZn(NO₃)₂.6H₂O, Al(NO₃)₃.9H₂O and La(NO₃)₃.6H₂O.
 13. The method of claim1, wherein step (d) is performed for a period of at least 1 hour at atemperature being comprised between 1° C. and 150° C.
 14. The method ofclaim 13, wherein step d) comprises maintaining the precipitate and themixture in contact with each other at a temperature between 1° C. and65° C.
 15. Lanthanide doped layered double hydroxides (Ln-doped LDHs),wherein a lattice parameter a₁₁₀ of a unit cell of a crystal structureof the lanthanide doped layered double hydroxides is at least 1.6%larger than a lattice parameter a₁₁₀ of a unit cell of a crystalstructure of a non-doped layered double hydroxide material.
 16. Thelanthanide doped layered double hydroxides according to claim 15,comprising brucite-like layers and interlayers between the brucite-likelayers, wherein at least 90% of anions in the interlayers are nitrateanions.
 17. The lanthanide doped layered double hydroxides according toclaim 16, wherein the lanthanide is doped into the brucite-like layer ofthe layered double hydroxides, such that the layered double hydroxidescomprise a non-segregated lanthanide phase.
 18. (canceled)
 19. Thelanthanide doped layered double hydroxides according to claim 15,wherein the lanthanide is La, Eu, or Tb. 20-22. (canceled)
 23. Themethod of claim 1, further comprising: using the lanthanide dopedlayered double hydroxides as sorbent for anions.
 24. The method of claim23, wherein the lanthanide doped layered double hydroxides are used assorbent for anions at a pH between 7 and
 14. 25. The method of claim 1,further comprising: using the lanthanide doped layered double hydroxidesas catalyst in a chemical reaction.
 26. The method of claim 25, whereinthe lanthanide doped layered double hydroxides are used as catalyst at apH between 7 and 14.